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TitleGround Motions of the 30 December 2015 M 4.7 Vancouver Island Earthquake: Attenuation and Site Response
AuthorJackson, F; Molnar, S; Ghofrani, H; Atkinson, G A; Cassidy, J F; Assatourians, K
SourceBulletin of the Seismological Society of America vol. 107, no. 6, 2017 p. 2903-2916,
Alt SeriesEarth Sciences Sector, Contribution Series 20160416
PublisherSeismological Society of America
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia; Western offshore region
AreaGeorgia Strait
Lat/Long WENS-124.0000 -122.0000 49.0000 48.0000
Subjectstectonics; geophysics; earthquakes; earthquake magnitudes; seismic arrays; seismic zones; seismic data; modelling; epicentres; pseudospectral acceleration (PSA); ground-motion prediction equation (GMPE)
Illustrationslocation maps; plots; graphs; tables; diagrams
ProgramAssessing Earthquake Geohazards, Public Safety Geoscience
Released2017 09 26
AbstractThe December 2015 moment magnitude (M) 4.7 Vancouver Island earthquake is one of the largest inslab earthquakes within Georgia Strait, British Columbia, Canada. Ground motions are retrieved and processed from all seismic stations within 500 km from the epicenter. Peak ground acceleration reached a maximum of 4.65 %g in Greater Victoria. We compare the observed horizontal-component 5% damped pseudo-acceleration (PSA) values with the in-slab GMPE suite used in the 2015 National Building Code of Canada (NBCC). PSA values from this event are significantly lower than the expected 2015 NBCC values for a M4.7 in-slab event highlighting poor GMPE scaling at this lower magnitude. We develop an event-specific GMPE to constrain the source and attenuation parameters in comparison with seismological models. Potential site effects are examined at strong-motion stations within 85 km of the epicenter. High amplification at 4-6 Hz is observed on both thick sediment sites and on the northern edge of the Fraser Delta, as observed in previous earthquakes. Gross generalizations of observed variable earthquake shaking are captured by current regional seismic microzonation maps; however, local discrepancies are present. This earthquake generated the first borehole array recordings obtained at depth in British Columbia. Recorded motions increase towards surface and are of similar or higher amplitude than at nearby surface stations. Amplification between top and bottom sensors is a consistent factor of 7-8 in all three arrays over a similar 40-45 m depth interval of Fraser delta sediments. Cross-correlation analysis determines vs estimates (< 350 m/s) consistent with Fraser delta sediments.
Summary(Plain Language Summary, not published)
This study examines earthquake site response and seismic attenuation across southwest BC using recordings of a widely-felt magnitude 4.7 earthquake of December 2015. This is the first significant earthquake recorded on our dense strong motion network in the Vancouver area that was deployed to examine variations in earthquake shaking on a wide variety of soil conditions (from rock, to firm soil, to thin soft soil, to thick soft soil). We find wave amplification factors of up to 7-8 in some areas of the Fraser River Delta. Some of Canada¿s most important infrastructure (e.g., ports, airports) is located here. We also compare our site response results to several thousand ¿felt shaking intensities¿ reported across the region. This will help us to better constrain earthquake shaking during future earthquakes and to improve our national building codes and standards.